Expense ratio
The yearly fee a fund charges, shown as a percentage of what you have invested in it. It comes out of the fund automatically.
You never get a bill for an expense ratio. The fund takes it out of its assets, which lowers your return a little every day.
Small differences add up over decades, because money paid in fees no longer compounds for you. Broad index funds often have some of the lowest expense ratios. You can find the expense ratio on the fund's fact sheet or prospectus.
Example
On $10,000, a 0.05% expense ratio costs $10,000 × 0.05% = $5 a year. A 1% expense ratio costs $10,000 × 1% = $100 a year.